The solid Earth is surrounded by a fluid envelope of atmosphere and ocean, a coupled system of direct interaction.

Unlike our Venusian and Martian neighbors, which have extremely high concentrations of carbon dioxide in their atmosphere, most of Earth’s carbon dioxide is stored in the ocean as carbonate rocks such as coral and shells of microscopic plankton or in vast deposits of limestone and marble.

The carbonate cycle is a complex dynamic equilibrium between carbon dioxide in the atmosphere and in seawater that controls the pH or acidity of the oceans, and the amount of carbonate deposits.

Carbon dioxide reacts with water to form a molecule of carbonic acid, which is a weak acid. About 3 out of every 100 carbonic acid molecules break apart to release a bicarbonate ion and a hydrogen ion. The presence of those excess hydrogen ions increases the acidity of the water.

In seawater a series of chemical reactions act as a buffer to keep the acidity of the seawater at nearly constant levels over time. Microscopic plankton use bicarbonate ions and calcium ions from sea salt to build shells, and coral use it to build reefs from calcium carbonate.

Increases in acidity inhibit formation of calcium carbonate. Extreme acidity can even cause existing calcium carbonate to dissolve, causing the water to absorb more carbon dioxide from the atmosphere.

Because the surface water is well mixed and eventually mixes with deep water, the effect is slow, but absorption of carbon dioxide will eventually make the water more acidic with a lower pH.

Approximately 45% of the carbon dioxide that has been emitted into the atmosphere since the beginning of the industrial era has been absorbed by the ocean, and the overall pH of the ocean has dropped to 8.10 from 8.18.

This doesn’t sound like much, but pH is a logarithmic scale, so a change of this size represents a 16% increase in hydrogen ions. Although calculations are not precise because of the number of variables involved, the best figures predict a change of 40% by the middle of the century, dropping the pH to 7.95.

Absorbing carbon dioxide from the atmosphere will cause the pH of ocean waters to become more acidic, which in turn will cause coral reefs to shrink, and cause a drop in carbonate-based plankton production.

This represents a potential environmental crisis as great or greater than global warming.

Coral reefs are one of the most biologically productive ecosystems on the planet, producing food for commercial fisheries as well as for blue-water fish such as swordfish and tuna.

Plankton produce about half of all atmospheric oxygen by photosynthesis. The other half comes from land-based photosynthesis, most of which is from tropical rainforests, which are being systematically deforested.

There is no way to know the exact effects of increased carbonic acid on plankton and coral because of the difficulty of cultivating them and running experiments in captivity.

One thing is certain: increased human output of carbon dioxide affects the entire fluid envelope, and it puts more at stake than just the greenhouse effect.

Richard Brill is a retired professor of science at Honolulu Community College. His column runs on the first and third Fridays of the month. Email questions and comments to brill@hawaii.edu.